1. Field of the Invention
This invention generally relates to methods for forming a calibration standard and calibration standards for inspection systems. Certain embodiments relate to calibration standards that include particles having a lateral dimension that is certified after deposition of the particles on a specimen.
2. Description of the Related Art
Monitoring and controlling semiconductor fabrication processes by inspection and metrology have become increasingly important in the successful fabrication of advanced semiconductor devices as the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and fabrication processes. For example, defects caused by particles on a surface of a wafer are responsible for a substantial portion of the yield loss of very large scale integrated circuits. Currently available systems that may be used for inspection include systems configured to detect particles or other defects on a specimen by light scattered from the specimen. In such systems, light illuminates the specimen at some incident angle. For a relatively smooth and flat surface, the light will reflect mostly specularly from the surface (i.e., the angle of reflected light equals the angle of incidence) and only a small portion of the incident light will be scattered away from the specular direction. If a defect is present, a larger portion of the incident light will be scattered away from the specular direction. Examples of defects that may be present on a specimen include, but are not limited to, particles, crystal-originated particles (COPs), surface roughness, ions, heavy metals, organic or inorganic layers, and subsurface imperfections.
Inspection systems are generally calibrated by measuring scatter from a standard source. A calibration standard may include polystyrene latex (PSL) spheres having a certified lateral dimension deposited on a specimen. Although PSL sphere contamination is generally not a problem in semiconductor manufacturing, PSL spheres were chosen as a scattering standard for inspection systems because of their spherical shape, well-known refractive index, and availability. PSL spheres are available in sizes typically ranging from tens of nanometers to tens of micrometers. The inspection system is calibrated using the measured response from a range of PSL sphere sizes that cover a portion of the range of the system. A relationship between detector response and PSL sphere diameter of sizes that were not calibrated may be produced by curve fitting, interpolation, or extrapolation of the data. Detector response can be converted into “PSL equivalent sizes.” After the inspection system is calibrated, the inspection system can be used to inspect specimens having an unknown number and various types of defects on the specimens. When the inspection system is used to inspect a specimen, a relationship can be used to convert PSL equivalent information to a defect size. Such a relationship can be determined from scattering models for the defects of interest and the PSL spheres.
There are several problems with PSL spheres as a calibration standard. One problem is accurately determining the size of the spheres. For example, the size of PSL spheres reported by a manufacturer may be different than the size of the PSL spheres determined with a differential mobility analyzer (DMA) or light scattering. Changes in the size of the PSL spheres may produce a substantial change in scattered light. Therefore, scattered light is an extremely sensitive measurement for particle size, but accurate sizing of the PSL sphere used for calibration is also extremely important. In addition, the PSL size information is used to determine sizes of the defects. Therefore, the accuracy of the PSL size will affect the accuracy of the defect sizes determined by the inspection system. Variations in the refractive index of PSL spheres can also cause changes in scattering levels and substantial errors in determining particle sizes.
Furthermore, as the size of defects that are being inspected decreases, the size of the PSL spheres used for calibration standards should also decrease. In this manner, the inspection system is calibrated at approximately the size of the defects. In addition, the accuracy requirements of such standards increases. For example, Semiconductor Equipment and Materials International (SEMI) has proposed that for the 130 nm technology generation “to reduce PSL sphere sizing uncertainty in the 65 nm to 200 nm range, the diameter distribution should have a full width at half maximum (FWHM)≦5%. In addition, it is desirable that the peak PSL diameter as deposited on the wafer has an expanded uncertainty at 95% confidence level as small as possible but not greater than 3% (2σ).” (“SEMI Draft Document 3094 New Standard for Specifying Scanning Surface Inspection Systems for Silicon Wafers for the 130-nm Technology Generation,” Jan. 21, 2002, page 13). However, currently available methods for producing a calibration standard for inspection systems can not meet the specification in terms of both size uncertainty and the ability to measure “as deposited on the wafer.” In addition, as the size of PSL spheres decreases, the accuracy of the size decreases, and the size distribution increases thereby further complicating meeting the specification.
Accordingly, it would be advantageous to develop a method for forming a calibration standard that meets the proposed specification for at least the 130 nm technology generation described above and that has a PSL sphere size certified after deposition of the PSL spheres on a specimen.